1. Field of the Invention
The invention relates to a method and an apparatus for detecting web breaks in web-fed rotary printing machines having a plurality of operating units which are arranged one after another and whose elements that interact directly or indirectly with the printing-material web can be driven synchronously.
2. Description of the Related Art
Until now, web break switches, as they are known, have been used for detecting web breaks. In most cases, these are optical sensors in the form of light barriers which, in the event of an impermissible deflection of an associated area of the printing-material web, output a signal. This is based on the finding that the printing-material web loses its tension in the event of a web break and leaves the normal transport plane. In order to assist this, blower nozzles are often associated with the optical sensors, which nozzles accelerate the deflection of the printing-material web which has lost its tension in the event of a web break.
The provision and installation of the aforementioned web break switches requires a comparatively high outlay. Since the optics of the light barriers can be impaired by their becoming dusty and soiled, high outlay on monitoring and maintenance is also required in order to ensure reliable serviceability. In addition, the known devices need a different amount of time to detect a web break, depending on their location.
The object of the present invention is to improve a method and an apparatus of the type mentioned at the beginning, with simple and cost-effective means, in such a way that high reliability and short reaction times are ensured.
In the method according to the invention, the torque on at least one element that belongs to each operating unit and interacts directly or indirectly with the printing material web is monitored continuously and a signal for a web break is derived from the occurrence of an abrupt torque change on the monitored element of at least one operating unit. In the apparatus according to the invention, a torque monitoring device is provided which has inputs for signals corresponding to the torque on at least one element that belongs to each operating unit and interacts directly or indirectly with the printing-material web and which, upon the occurrence of an abrupt torque change on at least one monitored element, generates an output signal associated with a web break.
Instead of sensors, the invention makes use of the torque profile which is characteristic in the event of a web break. This is based on the thought that, as long as the paper web does not have a break, the web forces which act upstream and downstream of a nip or cylinder gap through which the printing-material web passes cancel each other out. In this case, the drive devices output only the processing and flexing torques. In the event of a web break, the web force falls away in one or the other direction, so that the web forces no longer cancel each other out in relation to a nip or cylinder gap adjacent to the web break, and an additional torque acts on the associated drive device and manifests itself as an abrupt change, which can be interpreted as a web break. A further advantage is to be seen in the fact that the abrupt torque change on a monitored element at the same time also results in a reference to the point of the web break in the vicinity of this element. A further advantage is to be seen in the fact that torque monitoring can also be used to provide overload protection.
The abrupt torque change occurs simultaneously on two operating units that flank the location of the web break and have elements that roll on the printing-material web, a rise in torque resulting on the operating unit which is downstream with respect to the web break, and a fall in torque resulting on the operating unit which is upstream of the web break. Expediently, therefore, a signal for a web break can be derived from the simultaneous occurrence of opposite torque changes on two elements that belong to successive operating units and interact directly or indirectly with the printing-material web. In this way, it is extremely simply possible to distinguish the torque variation characteristic of a web break from torque changes which can be attributed to other disturbances, such as channel impacts, creases in the paper web, etc, and therefore to achieve particularly high reliability.
In the case of web-fed rotary printing machines in which each operating unit which has elements that roll on the printing-material web and in each case has at least one motor associated with it, the torque output by a motor to each such operating unit can advantageously be monitored continuously. This results in electrical variables which automatically correlate with the torque, which makes signal processing easier.
In a further development of the above measures, in the case of a web-fed rotary printing machine with individual drive to the elements that roll on the printing-material web, the torque output by a motor associated with an element directly involved in web transport is monitored continuously. In this case, the abrupt torque change appears particularly clearly, which makes monitoring easier and ensures the achievement of high reliability.